Hexafluorophosphate DPF6 (D=Li, Na, K, etc.) is used as the electrolytic liquid of a battery. Additionally, in the case of D=Ag, it has attracted attention as a counter ion that generates acid necessary for an initiation reaction in photo polymerization and a growth reaction. Furthermore, in the case of D=NH4, it is useful as a raw material that is used in the manufacturing of medicine intermediates.
Hexafluorophosphate DPF6 in a solid state is a compound that is generally unstable in water. When moisture exists in the reaction system, oxyfluorophosphate (DPOaFb) is produced as a by-product and hexafluorophosphate is decomposed into DF or DF.c (HF). Among those, especially when lithium hexafluorophosphate, silver hexafluorophosphate, etc. are left in air, they decomposes gradually while generating white smoke (POF3 and HF) due to a very small amount of moisture. This decomposition occurs even with only a few ten ppm of moisture, and the moisture changes into 2 times equivalent or more of an acid component (HF, HPO2F2, H2PO3F, etc.)
For this reason, conventionally, hexafluorophosphate is generally obtained by producing phosphorous pentafluoride (PF5) by reacting anhydrous hydrogen fluoride with phosphorous halide, for example, phosphorous pentachloride (PCl5), and further by reacting the generated PF5 with metal fluoride (DF or DF.c (HF)) in anhydrous hydrogen fluoride.
However, because a solid having moisture absorbing property and smoke-generating property, so-called phosphorus pentachloride, is used in this method and a large amount of hydrogen chloride gas is generated during the reaction, this method has a disadvantage that workability is poor such that treatment of the generated gas has to be performed.
On the other hand, it is also known that the disaggregated hexafluorophosphate anion (PF6−) is also stable in an aqueous solution. This is because a complex with a water molecule is formed where a stable coordination compound such as water exists. However, it is difficult to extract also the hexafluorophosphate anion that is stable in a solution due to water existing in a solution as hexafluorophosphate that is an anhydrous solid.
For example, NaPF6 exists as a stable PF6− in a solution when it is dissolved in water. However, in the case of extracting this as a crystal with a method such as recrystallization, NaPF6 deposits in a state of NaPF6.H2O that is a hydrate, not as NaPF6 that is an anhydride. Furthermore, when heating or drying under reduced pressure is performed to dehydrate water content of this NaPF6.H2O, a part of hexafluorophosphate is decomposed into oxyfluorophosphate. The same decomposition also occurs in NH4PF6, AgPF6, and LiPF6, and especially it is remarkable in the case of LiPF6 and AgPF6, which are decomposed into oxyfluorophosphate even when a small amount of a hydrate exists. For this reason, a report that LiPF6, NaPF6, NH4PF6, and AgPF6 have been synthesized in an aqueous solution system has not existed yet.
As described above, although hexafluorophosphate DPF6 is unstable in water, in the case of D=K (potassium) (KPF6), a crystal is successfully extracted as a solid even under the existence of a small amount of water. For example, the following Patent Document 1 describes a manufacturing method of KPF6 by manufacturing a hexafluorophosphoric acid solution with reaction of anhydrous hydrogen fluoride with phosphorus pentaoxide, and then by reacting with a potassium salt such as potassium fluoride and potassium hydroxide.
However, with this method, because the reaction of phosphorous pentaoxide with anhydrous hydrogen fluoride is violent, it is very dangerous and control of the reaction is extremely difficult. Furthermore, it has a disadvantage that workability is poor such that phosphorus pentaoxide that has very strong moisture absorbing property is handled.
Then, in the following Patent Document 2, potassium hexafluorophosphate is synthesized by reacting with a potassium compound of phosphoric acid (potassium dihydrogen phosphate or potassium polyphosphate) using hydrofluoric acid of 60 to 97% by weight even without using anhydrous hydrogen fluoride. However, it is described in this method that although the reaction is not as violent as phosphorus pentaoxide, the temperature of the solution increases as the reaction proceeds when adding a potassium salt into hydrofluoric acid. Furthermore, a synthesis of rubidium hexafluorophosphate and cesium hexafluorophosphate may be possible if this method is applied. However, in this case, because rubidium dihydrogen phosphate, cesium dihydrogen phosphate, rubidiumpolyphosphate, and cesium polyphosphate cannot be easily obtained, it is hard to say that it is a practical method.
On the other hand, hexafluorophosphoric acid (HPF6) is synthesized as in the following chemical reaction Formula 4 or 5 according to the following Patent Document 3. This is because water is coordinated in HPF6 and HPF6 is stabilized.H3PO4+6HF→HPF6+4H2O  (Formula 4)P2O5+12HF→2HPF6+5H2O  (Formula 5)
However, the extraction of HPF6 as a crystal (a solid) is not performed in this technique, and it is discussed only as a solution of HPF6.
Furthermore, in the following Patent Document 4, PF5 gas is generated by utilizing HPF6. However, the extraction of HPF6 as a crystal (a solid) is not performed in the technique as well.
Furthermore, in the following Patent Document 3, 4, etc., there is a description of HPF6.6H2O, HPF6.4H2O, and HPF6.2H2O. However, it is not described that these are a crystal (a solid) and it can be read from the description that they are a coordination number in the solution. That is, it is considered to be very difficult to easily extract hexafluorophosphoric acid as a crystal (a solid) from a hexafluorophosphate anion (PF6−) solution.
[Patent Document 1] U.S. Pat. No. 2,488,299
[Patent Document 2] Japanese Examined Patent Publication No. 5-72324
[Patent Document 3] Japanese Unexamined Patent Publication No. 2004-75413
[Patent Document 4] Japanese Translation of a PCT International Patent Application No. 2005-507849